In a computer or telecommunication system, an interface connecting to the external is generally designed as a bus for transmitting various data. The hardware interface is fabricated to be an expandable slot, i.e., a so-called edge connector. The slot is attached to a motherboard. A modular daughter board, such as an interface card or memory card, can be inserted into the slot, so as form a connection interface. The edge connector generally connects a motherboard and a daughter board in the following manner. Through holes for connecting the connectors are formed on the motherboard. A conductive metal layer is plated to the inner surface and the periphery of the through holes, and the circuits to be connected to the bus on the motherboard are connected to the corresponding through holes. Pins of the edge connector pass through the through holes of the motherboard and are temporarily retained on the motherboard. The pins are then firmly retained on the motherboard by soldering or other processes, so as to achieve electrical connection with the motherboard.
The daughter board includes various electronic elements and circuit structures required for achieving the interface function, and “gold fingers,” i.e., a row of conductive pads, in the shape of rectangle or ellipse, connected to the circuits of the daughter board, are fabricated on the end or edge portion of the daughter board for connecting to the edge connector. The end or edge portion with gold fingers of the daughter board is inserted into the slot of the edge connector. Two inner sides of the slot contain contact areas of conductors; the other ends of the conductors are pins of the connector to be soldered on the motherboard. The gold fingers contact the contact areas of the conductors of the connector correspondingly, so that the circuits of the motherboard and the daughter boards are communicated.
The electrical connector is mainly used for transmitting signals completely and correctly, and is a typical passive element. In recent years, the speed of central processing unit (CPU) for computers is improved continuously, from the earliest 33 MHz, 66 MHz, to Pentium III 500 MHz and to the latest Pentium IV 3.06 GHz. As such, the electronic signal transmission speeds of main board and computer peripherals must be increased accordingly, so as to match the processing speed of the CPU.
Signal transmission through a connector can be classified into two modes, namely single-ended signal and differential mode signal. The single-ended signal means that only one conductor is used when transmitting one signal between two electronic elements or devices that are connected. The transmission of a differential mode signal requires two matching conductors, so as to transmit the signals back and forth between two electronic elements or devices that are connected.
The differential mode signals transmitted on two conductors are two complementary signals, i.e., having the same amplitude but opposite polarities (with a phase difference of 180 degrees). In a high-speed transmission environment, a better electrical characteristic is obtained by differential mode signal transmission. The time sequence and response capability required by the system can be easily achieved, thereby the probability of the system's misjudging or missing part of the data can be reduced. Therefore, in practice, when an edge connector is used to connect various interface cards and memory modules to the motherboard, each pair of conductors (generally referred to as “contacts”) on one side of the connector is mostly used to transmit signals under the differential mode.
Although better electrical characteristics of a connection system can be obtained by using differential mode signal transmission, the electrical characteristics of a pair of differential mode signals with opposite polarities is affected by the design factors of the connector, such as the material and shape of the conductor, the arrangements of the conductors in relation to each other, as well as the arrangement of the conductors with the insulative material. Particularly, as computers and communication apparatus are getting smaller, the structure of the electronic connector also becomes more and more impact, e.g. with the distance between the conductors of the connector greatly reduced, and the density of the pins increased. These changes, however, aggravates the problems related to signal transmission with high speed or high frequency, such as impedance, cross talk, propagation delay, attenuation, skew and rise time degradation. Therefore, obtaining desired performance of the system with a appropriate connectors has become a challenge to the industry.